1. Field of the Invention
The invention is related to a method of producing an organic silver salt particle dispersion and to a high-quality photothermographic material with excellent coated surface state.
2. Description of the Related Art
Reduction of waste solutions to be treated has been strongly desired in recent years in the medical field from the viewpoints of environmental protection and space saving. Under such circumstances, technologies on photosensitive photothermographic photographic materials for medical diagnosis and photography which can be exposed to light efficiently with a laser image setter or a laser imager, and can form a clear black image having high resolution and sharpness have been demanded. With these photosensitive photothermographic photographic materials, it is possible to supply to customers a heat development treatment system which has eliminated the necessity of using solvent system processing chemicals, and is simpler and does not impair the environment.
Similar requirements also exist in the field of general image forming materials. However, the image for medical use is required to have a high image quality excellent in sharpness and graininess, because fine details of the image are required. In addition, the medical image is characterized by preferably exhibiting a blue black image tone from the viewpoint of ease of medical diagnosis. Currently, various hard copy systems utilizing pigments or dyes such as inkjet printers and apparatuses for electrophotography are prevailing as general image forming systems. However, there is no system which is satisfactory as a medical image-output system.
A thermal image formation system utilizing an organic silver salt is described in a large number of documents. In particular, the photothermographic material generally has an image-forming layer in which a catalytically active amount of a photocatalyst (e.g., silver halide), a reducing agent, a reducible silver salt (e.g., organic silver salt), and, if required, a toning agent for controlling the color tone of silver are dispersed in a binder matrix. The photothermographic materials are, after being imagewise exposed, heated to a high temperature (for example, to 80° C. or higher) to form black silver images through the oxidation-reduction reaction between the silver halide or the reducible silver salt (which functions as an oxidizing agent) and the reducing agent therein. The oxidation-reduction reaction is accelerated by the catalytic action of the latent image of the silver halide generated through exposure. For this reason, the black silver images are formed in the exposed areas. Fuji Medical Dry Imager FM-DP L has been distributed as a medical image formation system using a photothermographic material.
In the production of a thermal image-forming system using an organic silver salt, the following exemplary methods can be conducted: in a method, the image-forming material is formed by coating operations using a solvent; in another method, a coating liquid is coated and dried which contains polymer fine particles as a main binder dispersed in water. The latter method is advantageous because: processes for collecting solvents or the like are unnecessary and the production facility can be simple, the latter method imposes less environmental burden, and the latter method is suitable for large-scale production. However, the coating liquid used in the latter method does not have setting property. Therefore, the film is affected by the drying wind after application of the coating liquid, and drying unevenness easily occurs.
It has been proposed (for example in U.S. Pat. Nos. 6,630,291 and 6,713,241, the disclosures of which are incorporated herein by reference) to use a hydrophilic binder such as gelatin as the binder. However, the resultant image-forming material has poor thermal development activity and fogging inevitably occurs when the activity is heightened to obtain a sufficient image. The image-forming material of this type has not been put into practical use.
In a photothermographic material, the film has to contain chemical components necessary for image formation even before image formation. Accordingly, the chemical components affect the storage stability of the photothermographic material before use. In addition, after image formation through thermal development, the chemical components remain in the film in the form of an unreacted substance or a reaction product. Accordingly, the chemical components affect the transparency of the film and the tone of the image, and adversely affect the storage stability of the image. The problems related to the storage stability are more remarkable when the image-forming layer contains a hydrophilic binder, whereby methods for improving such properties have been desired.